1. Field of the Invention
The present invention generally relates to wireless communications. More specifically, the present invention relates to multiple-input multiple-output (MIMO) wireless antennas.
2. Description of the Prior Art
In wireless communications systems, there is an ever-increasing demand for higher data throughput and a corresponding drive to reduce interference that can disrupt data communications. For example, a wireless link in an Institute of Electrical and Electronic Engineers (IEEE) 802.11 network may be susceptible to interference from other access points and stations, other radio transmitting devices, and changes or disturbances in the wireless link environment between an access point and remote receiving node. In some instances, the interference may degrade the wireless link thereby forcing communication at a lower data rate. The interface may, however, be sufficiently strong as to disrupt the wireless link altogether.
One solution is to utilize a diversity antenna scheme. In such a solution, a data source is coupled to two or more physically separated omnidirectional antennas. An access point may select one of the omnidirectional antennas by which to maintain a wireless link. Because of the separation between the omnidirectional antennas, each antenna experiences a different signal environment and corresponding interference level with respect to the wireless link. A switching network couples the data source to whichever of the omnidirectional antennas experiences the least interference in the wireless link.
Diversity schemes are generally lacking in that typical omnidirectional antennas are vertically polarized. Vertically polarized radio frequency energy does not travel as efficiently as horizontally polarized energy with respect to a typical wireless environment (e.g., a home or office). Omnidirectional antennas also generally include an upright ‘wand’ attached to the access point. These wands are easily susceptible to breakage or damage. Omnidirectional antennas in a diversity scheme, too, may create interference amongst one another or be subject to the same interference source due to their physical proximity. As such, a diversity antenna scheme may fail to effectively reduce interference in a wireless link.
An alternative to a diversity antenna scheme involves beam steering of a controlled phase array antenna. A phased array antenna includes multiple stationary antenna elements that employ variable phase or time-delay control at each element to steer a beam to a given angle in space (i.e., beam steering). Phased array antennas are prohibitively expensive to manufacture. Phased array antennas, too, require a series of complicated phase tuning elements that may easily drift or otherwise become maladjusted over time.
Another attempt to improve the spectral efficiency of a wireless link includes the use of MIMO antenna architecture in an access point and/or receiving node. In a typical MIMO approach, multiple signals (two or more radio waveforms) are generated and transmitted in a single channel between the access point and the remote receiving node. FIG. 1 illustrates an exemplary access point 100 for a MIMO antenna system having two parallel baseband-to-RF transceiver (“radio”) chains 110 and 111 as may be found in the prior art.
Data received into the access point 100 from, for example, a router connected to the Internet is encoded by a data encoder 105. Encoder 105 encodes the data into baseband signals for transmission to a MIMO-enabled remote receiving node. The parallel radio chains 110 and 111 generate two radio waveforms by digital-to-analog (D/A) conversion and upconversion. Upconversion may occur through the use of an oscillator driving a mixer and filter.
Each radio chain 110 and 111 in FIG. 1 is connected to an omnidirectional antenna (120 and 121, respectively). As with a diversity scheme, the omnidirectional antennas 120 and 121 may be spaced as far apart as possible from each other or at different polarizations and mounted to a housing of the access point 100. The two radio waveforms are simultaneously transmitted, affected by various multipath perturbations between the access point 100 and the MIMO-enabled remote receiving node, and then received and decoded by appropriate receiving circuits in the remote receiving node.
Prior art MIMO antenna systems tend to use a number of whip antennas for a number of transmission side radios. The large number of whip antennas used in a prior art MIMO antenna system not only increase the probability that one or more of the antennas may be damaged during use but also creates unsightly ‘antenna farms.’ Such ‘farms’ are generally unsuitable for home or business applications where access points are generally desired, if not needed, to be as small and unobtrusive as possible.
There remains a need in the art for wireless communication providing increased data throughput and reduced interference. An access point offering said benefits should do so without sacrificing corresponding benefits related to size or manageability of the access point.